Liquid crystal cumulative dosimeter

ABSTRACT

Cumulative temperature dosimeter utilizes a temperature dependent reaction in a cholesteric liquid crystal composition to produce a color change indicative of temperature exposure over a period of time. 
     Typically, the composition includes, as a solvent, a cholesteric liquid crystal, such as a mixture of p-methoxy-p&#39;-n-butyl-azoxybenzene, cholesteryl nonanoate, cholestyl chloride, and cholesteryl oleyl carbonate. One or more solutes are included, the solutes being reactive at a rate which is temperature dependent, to cause either generation or consumption of mesogenic compounds. In turn, the pitch of the solvent, observable as a visible color change, is affected in proportion to the cumulative temperature exposure of the composition.

This invention pertains to liquid crystal compositions suitable for usein cumulative dosimeters and to cumulative dosimeters based on suchcompositions. More specifically, this invention pertains to compositionsincluding a cholesteric liquid crystal compound, the light reflectingproperties of which are changed by a reaction which is dependent uponthe cumulative exposure of the composition to temperature within alimited range.

Heat sensitive compositions are well known. In many cases, color changesare effected in such compositions which are indicative of the exposureof the compositions to a particular temperature. Usually, thesecompositions are reversible. Often, the change reflects a physicalreaction, such as fusion or melting, although the rate of reaction issometimes tailored, such that a certain lag is built into the indicator.Liquid crystals have been used in many instances in indicatorcompositions of these types.

But indicators, even irreversible ones, which indicate only that acertain condition, such as a specific temperature has been reached, areof limited value where it is important to know that a temperaturecondition outside of some limit has prevailed for several hours. In thecase of frozen foods, for example, it is not enough however to know thatthe surface temperature of a frozen food package has exceeded 0° C for ashort period of time. A more realistic measure of the acceptability ofstorage conditions of such a frozen food package would be a cumulativeindication of the exposure of the package to elevated temperature.

It is the general object of the present invention to providecompositions wherein an observable change depends on the cumulativeexposure of a composition or product, to a temperature within aparticular temperature range.

A more specific object of this invention is to provide such compositionsbased on cholesteric liquid crystals, and most specifically to providesuch compositions wherein the change is observable as a color change.

These and other objects which will be apparent in the course of thesubsequent description of this invention are met by a compositionincluding a cholesteric liquid crystal compound with one or more of theconstituents of the composition being adapted to enter into a chemicalreaction, the rate of which is directly proportional to the exposure ofthe composition to temperature. The light reflecting properties of thecomposition are changed in proportion to the degree of completeness ofthe reaction, preferably by effecting a color change of the composition.

Preferably also the composition is packaged in a manner to facilitateits use as an indicator of the cumulative dosage of some product to heatin a particular temperature range. More preferably, the composition ofthis invention is adapted to sense a cumulative dosage of frozenproducts to temperature above the desired storage temperature, forexample with a reaction adapted to reach equilibrium within a range ofexposure from 5° C for 100 to 1000 hours, to 25° C for 1 hour to 100hours.

This invention may be better understood by reference to the followingdetailed description thereof, taken in conjunction with the appendedclaims.

Generally, the light reflecting properties, particularly the color, of acholesteric liquid crystal compound, are readily changed, either by somechemical change in the liquid crystal or by a reaction involving theliquid crystal to produce some other compound which affects the colorthereof, or by the reaction of extraneous materials, such as a solute ordiluent, which affects the color of the liquid crystals differently thanthe starting compounds in the reaction. To the extent such a reaction israte dependent on exposure to energy in some form, such as temperature,the color change observable in the cholesteric liquid crystals iscumulatively indicative of the time and intensity or level of the energyinput to which the reaction is sensitive.

In the present invention, compositions, yielding such cumulativeindications of temperature exposure, are based on cholesteric liquidcrystal solvents chosen such that the wavelength of maximum reflectionof the cholesteric is in the visible region of the spectrum and suchthat the temperature dependence of the wavelength of maximum reflectionof the cholesteric is negligible at the temperatures involved in thereaction. In this cholesteric solvent solutes are included. Thesesolutes are reactive at a rate which is temperature dependent, theirreaction either producing or consuming an optically active mesogenicmolecule. The generation or consumption of such a molecule in thesolvent perturbs the pitch of the solvent to cause either a red or blueshift in the wavelength of maximum reflection.

As one example of the composition within the scope of the presentinvention, a relatively temperature-insensitive cholesteric liquidcrystal is combined with N-(p-methoxybenzylidene)-p-n-butylaniline(MBBA) and (-)-α-phenethylamine. These compounds react at a ratedependent on temperature to produce ##STR1## The products of thisreaction have a much higher degree of helical twisting power than thereactants and, as a result, the composition changes color as thereaction goes toward completion. As a specific example of onecholesteric liquid crystal which is relatively temperature-insensitivein its light reflecting properties and which is suitable as a solvent inthe compositions of the present invention, a mixture of 0.9 parts byweight of cholesteryl nonanoate, 1.1 parts by weight of cholesterylchloride, and 2 parts by weight of cholesteryl oleyl carbonate is mixedwith 40% by weight of Nematic Phase V (a product consisting of theisomers of p-methoxy-p'-n-butylazoxybenzene, trade name Nematic Phase Vof E. Merck, Darmstadt, West Germany) to yield a blue cholestericsolvent. The same mixture of cholesterics is mixed with 55% by weight ofNematic Phase V to yield a red solvent.

A variety of other cholesteric solvents may also be used, such asmixtures of left-handed cholesteric compounds such as cholesterylnonanoate, hexanoate, decanoate, etc. with right-handed cholestericcompounds such as cholesteryl chloride, bromide, iodide or nitrate,blended with cholesteryl oleyl carbonate or cholesteryl-2-(ethoxyethoxy)ethyl carbonate or any low temperature mesomorphic carbonate ester ofcholesterol. To this cholesteric mixture may be added nematogenicmaterials such as Nematic Phase V, a p-alkyl-p'-cyano biphenyl or anynematogen having a nematic range close to room temperature.

A variety of other compounds may be included in such cholestericsolvents either to generate or consume optically active mesogenicmolecules. For example aldehydeamine condensations, such as ##STR2## (Rand R" are alkyl or alkoxy with 1 to 8 carbon atoms and R' is analkylene group, e.g. --(CH₂)--, with from 1 to 18 carbon atoms) whereone of the components of the reaction is optically active. Two specificexamples of such reactions are ##STR3##

Another class of reaction which may be used in this manner are amineexchange reactions of Schiff bases, wherein a mesogenic Schiff base isreacted with practically any amine. In this class of reactions, both theSchiff base and the amine may be optically active. Examples of suchreactions are

Class B: Amine exchange reactions of Schiff bases. ##STR4##

Reactants which generate a mesogenic benzoate ester may also be used inthe compositions of this invention. In general, these reactions proceedas follows: ##STR5##

Compositions based on the foregoing are designed and calibrated toeffect a characteristic change in light reflecting properties dependenton the exposure of the reactants to temperature and indicative of theexposure of the reactants to temperature within a specific temperaturerange over a period of time. A color change is thus exhibitedindicating, for example, that the composition has been exposed to atemperature in the range of 0° -25° C for a period of several hours, thedegree of the color change being proportional to the degree ofcompletion of the reaction.

In a typical application of the composition of the present invention, apharmaceutical product may be packaged with the pod of a cholestericliquid crystal containing chemical solutes which react at a reasonablerate only at temperatures greater than 25° C but which react at slowerrates at temperatures up to 25° C. The cholesteric is blended so thatits normal color is, for example, red at temperatures between 0° C and25° C with a shift in color brought about by reaction of the solutes, toblue, for example, The proposed storage temperature of thepharmaceutical is 0° C or below, at which temperature the reaction rateof the solutes in the cholesteric liquid crystal is substantially zero.Further, the solute reaction is controlled by means of concentration ofreactants so that the reaction will go substantiallly to completionwithin a range of exposure of from 5° C for 100 to 1000 hours to 25° Cfor 1 to 100 hours.

Preferably also, the indicator composition of this invention is packagedin a pod or container with a transport section which may be mounted onthe surface of the product to be monitored for temperature dosage. Stillmore preferably, the composition containing pod may include an elongatedsection to extend inwardly in the product so as to sense the innertemperature in the case of a temperature dosimeter.

A color indicator may also be included on the container or near thesensing composition for comparison to the color of the composition whichis indicative of the dosage levels of concern.

While this invention has been described with reference to particularembodiments thereof, it should be understood that it is not limitedthereto and that the intended claims are intended to be construed tocover such equivalent variations and modifications of this inventionwhich may be made by those skilled in the art without departing from thetrue spirit and scope thereof.

I claim:
 1. A cholesteric liquid crystal composition comprising acholesteric liquid crystal solvent and reactive solutes adapted to enterinto a chemical reaction, said liquid crystal solvent having awavelength of maximum reflection the temperature dependence of which isnegligible at the temperatures involved in the reaction, the rate ofsaid reaction being temperature dependent, the products of said reactionbeing adapted to cause a characteristic change in the light reflectingproperties of said composition, the degree of said change beingcumulatively indicative of the exposure of said composition totemperature outside of a preselected limit.
 2. A composition, as recitedin claim 1, wherein said different light reflecting properties areobservable as a color change in said composition.
 3. A composition, asrecited in claim 1, wherein said solutes are reactive at a temperaturedependent rate to generate or consume mesogenic molecules.
 4. Acomposition, as recited in claim 3, wherein said cholesteric liquidcrystal solvent comprises a mixture of cholesteryl nonanoate,cholesteryl chloride, cholesteryl oleyl carbonate, and the isomers ofpara-methoxy-p'-n-butylazoxybenzene.
 5. A composition, as recited inclaim 4, wherein said mixture consists of 0.9 parts by weight ofcholesteryl nonanoate, 1.1 parts by weight of cholesteryl chloride, and2 parts by weight of cholesteryl oleyl carbonate, combined with 40% byweight of a sub-mixture of the isomers ofparamethoxy-p'-n-butylazoxybenzene.
 6. A composition as recited in claim4, wherein said mixture consists of 0.9 parts by weight of cholesterylnonanoate, 1.1 parts by weight of cholesteryl chloride, and 2 parts byweight of cholesteryl oleyl carbonate, combined with 55% by weight of asub-mixture of the isomers of paramethoxy-p'-n-butylazoxybenzene.
 7. Acomposition, as recited in claim 1 wherein said composition comprises amixture of left-handed cholesteric compounds selected from the groupconsisting of cholesteryl nonanoate, cholesteryl hexanoate, cholesteryldecanoate or any other straight chain cholesteryl C₁ to C₂₀ alkanoateand right-handed cholesteric compounds selected from the groupconsisting of cholesteryl chloride, cholesteryl bromide, cholesteryliodide and cholesteryl nitrate, in combination with any low temperaturemesomorphic carbonate ester of cholesterol.
 8. A composition, as recitedin claim 7, wherein said low temperature mesomorphic carbonate ester ofcholesterol is cholesteryl oleyl carbonate orcholesteryl-2-(ethoxyethoxy) ethyl carbonate.
 9. A composition, asrecited in claim 3, wherein said reactive solutes enter into a mesogenicreaction selected the group consisting of aldehyde-amine condensations,Schiff base-amine reactions or benzoate ester formations.
 10. Acomposition, as recited in claim 9, wherein said reactive solutes arep-ethoxybenzaldehyde and (+) or (-) α-phenethylamine.
 11. A composition,as recited in claim 9, wherein said reactive solutes are ##STR6##
 12. Acomposition, as recited in Claim 9, wherein said reactive solutes areN-(p-methoxybenzylidene)-p-n-butyl-aniline and (-)-α-phenethylamine. 13.A composition, as recited in claim 9, wherein said reactive solutes are##STR7##
 14. A composition, as recited in claim 3, wherein said reactionrate is dependent on and proportional to exposure of said composition toa temperature above some predetermined maximum.
 15. A container, havingat least one transparent section to permit observation of the color of amaterial disposed therein, said container containing a composition asrecited in claim
 1. 16. A container with composition therein, as recitedin claim 15, said container having a shape adapted to extend inwardlyinto a product to sense the cumulative temperature exposure thereof. 17.A container with composition therein, as recited in claim 15, whereinsaid container includes a color indicator for comparison to saidcomposition to determine the cumulative dosage sensed thereby.
 18. Amethod of determining the cumulative time of exposure of an indicatorelement to temperatures above a preselected limit said method comprising(1) providing a cholesteric liquid crystal composition comprising acholesteric liquid crystal solvent and reactive solutes adapted to enterinto a chemical reaction at temperatures above said preselectedtemperature limit, said liquid crystal solvent having a wavelength ofmaximum reflection the temperature dependence of which is negligible atthe temperatures involved in the reaction, the rate of said reactionbeing temperature dependent, and (2) after a period of time, observingany change in the optical properties of the cholesteric liquid crystalcomposition.